Дисертації з теми "Gradient-free"
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1
Werner, Tino [Verfasser], Peter [Akademischer Betreuer] Ruckdeschel, and Matthias [Akademischer Betreuer] Schmid. "Gradient-Free Gradient Boosting / Tino Werner ; Peter Ruckdeschel, Matthias Schmid." Oldenburg : BIS der Universität Oldenburg, 2020. http://d-nb.info/120419968X/34.
2
Walton, Sean Peter. "Gradient free optimisation in selected engineering applications." Thesis, Swansea University, 2013. https://cronfa.swan.ac.uk/Record/cronfa43116.
3
Yucelen, Tansel. "Advances in adaptive control theory: gradient- and derivative-free approaches." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/43731.
Анотація:
In this dissertation, we present new approaches to improve standard designs in adaptive control theory, and novel adaptive control architectures.
We first present a novel Kalman filter based approach for approximately enforcing a linear constraint in standard adaptive control design. One application is that this leads to alternative forms for well known modification terms such as e-modification. In addition, it leads to smaller tracking errors without incurring significant oscillations in the system response and without requiring high modification gain. We derive alternative forms of e- and adaptive loop recovery (ALR-) modifications.
Next, we show how to use Kalman filter optimization to derive a novel adaptation law. This results in an optimization-based time-varying adaptation gain that reduces the need for adaptation gain tuning.
A second major contribution of this dissertation is the development of a novel derivative-free, delayed weight update law for adaptive control. The assumption of constant unknown ideal weights is relaxed to the existence of time-varying weights, such that fast and possibly discontinuous variation in weights are allowed. This approach is particularly advantageous for applications to systems that can undergo a sudden change in dynamics, such as might be due to reconfiguration, deployment of a payload, docking, or structural damage, and for rejection of external disturbance processes.
As a third and final contribution, we develop a novel approach for extending all the methods developed in this dissertation to the case of output feedback. The approach is developed only for the case of derivative-free adaptive control, and the extension of the other approaches developed previously for the state feedback case to output feedback is left as a future research topic.
The proposed approaches of this dissertation are illustrated in both simulation and flight test.
4
Harrison, Stephen C. W. "Free field modeling of a MEMS-Based pressure gradient microphone." Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Dec/09Dec%5FHarrison.pdf.
Анотація:
Thesis (M.S. in Engineering Acoustics and M.S. in Applied Physics)--Naval Postgraduate School, December 2009.Thesis Advisor(s): Karunasiri, Gamani; Baker, Steven. "December 2009." Description based on title screen as viewed on January 26, 2010. Author(s) subject terms: MEMS, Model, Simulation, Pressure Gradient, Particle Velocity, Directional, Microphone, SOMSOL, Acoustic. Includes bibliographical references (p. 51-52). Also available in print.
5
Nutini, Julie Ann. "A derivative-free approximate gradient sampling algorithm for finite minimax problems." Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/42200.
Анотація:
Mathematical optimization is the process of minimizing (or maximizing) a function. An algorithm is used to optimize a function when the minimum cannot be found by hand, or finding the minimum by hand is inefficient. The minimum of a function is a critical point and corresponds to a gradient (derivative) of 0. Thus, optimization algorithms commonly require gradient calculations. When gradient information of the objective function is unavailable, unreliable or ‘expensive’ in terms of computation time, a derivative-free optimization algorithm is ideal. As the name suggests, derivative-free optimization algorithms do not require gradient calculations. In this thesis, we present a derivative-free optimization algorithm for finite minimax problems. Structurally, a finite minimax problem minimizes the maximum taken over a finite set of functions. We focus on the finite minimax problem due to its frequent appearance in real-world applications. We present convergence results for a regular and a robust version of our algorithm, showing in both cases that either the function is unbounded below (the minimum is −∞) or we have found a critical point. Theoretical results are explored for stopping conditions. Additionally, theoretical and numerical results are presented for three examples of approximate gradients that can be used in our algorithm: the simplex gradient, the centered simplex gradient and the Gupal estimate of the gradient of the Steklov averaged function. A performance comparison is made between the regular and robust algorithm, the three approximate gradients, and the regular and robust stopping conditions. Finally, an application in seismic retrofitting is discussed.
6
Refsnæs, Runar Heggelien. "Matrix-Free Conjugate Gradient Methods for Finite Element Simulations on GPUs." Thesis, Norges Teknisk-Naturvitenskaplige Universitet, Institutt for fysikk, 2010. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-10826.
Анотація:
A block-structured approach for solving 2-dimensional finite element approximations of the Poisson equation on graphics processing units(GPUs) is developed. Linear triangular elements are used, and a matrix-free version of the conjugate gradient method is utilized for solving test problems with over 30 million elements. A speedup of 24 is achieved on a NVIDIA Tesla C1060 GPU when compared to a serial CPU version of the same solution approach, and a comparison is made with previous GPU implementations of the same problem.
7
Fejes, Niklas. "High Performance Multi-Objective Voyage Planning Using Local Gradient-Free Methods." Thesis, Uppsala universitet, Avdelningen för beräkningsvetenskap, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-305582.
Анотація:
A number of parallel gradient-free local optimization methods are investigated in application to problems of voyage planning for maritime ships. Two optimization algorithms are investigated, a parallel version of the Nelder-Mead Simplex method and the Subplex method with Nelder-Mead Simplex as its inner solver. Additionally, two new formulations of the optimization problem are suggested which together with an improved implementation of the objective function increases the overall performance of the model. Numerical results show the efficiency of these methods in comparison with the earlier introduced Grid search method and solvers from an open-source optimization library.
8
Li, Futong. "Global Optimization Techniques Based on Swarm-intelligent and Gradient-free Algorithms." Thesis, Université d'Ottawa / University of Ottawa, 2021. http://hdl.handle.net/10393/42307.
Анотація:
The need for solving nonlinear optimization problems is pervasive in many fields. Particle swarm optimization, advantageous with the simple underlying implementation logic, and simultaneous perturbation stochastic approximation, which is famous for its saving in the computational power with the gradient-free attribute, are two solutions that deserve attention. Many researchers have exploited their merits in widely challenging applications. However, there is a known fact that both of them suffer from a severe drawback, non- effectively converging to the global best solution, because of the local “traps” spreading on the searching space. In this article, we propose two approaches to remedy this issue by combined their advantages.
In the first algorithm, the gradient information helps optimize half of the particles at the initialization stage and then further updates the global best position. If the global best position is located in one of the local optima, the searching surface’s additional gradient estimation can help it jump out. The second algorithm expands the implementation of the gradient information to all the particles in the swarm to obtain the optimized personal best position. Both have to obey the rule created for updating the particle(s); that is, the solution found after employing the gradient information to the particle(s) has to perform more optimally.
In this work, the experiments include five cases. The three previous methods with a similar theoretical basis and the two basic algorithms take participants in all five. The experimental results prove that the proposed two algorithms effectively improved the basic algorithms and even outperformed the previously designed three algorithms in some scenarios.
9
Cantergiani, Elisa. "Mechanical Properties of Functionally Graded Materials: Carbon Gradient inside Interstitial Free Steel." Thesis, Université d'Ottawa / University of Ottawa, 2016. http://hdl.handle.net/10393/34314.
Анотація:
In the last decade aluminium started to be considered as an alternative to steel to produce car body panels, especially considering the strict demands to decrease fuel consumption which require vehicle weight reduction. In order to keep their leading role, steel companies have to produce stronger materials to reduce the thickness of steel sheets used in cars and are now considering non-conventional steel making processes.
The purpose of this PhD research was to investigate the possibility of strengthening thin sheets of interstitial free steel (IF steel) by using carbon rich films deposited on the steel surface using Physical Vapour Deposition (PVD). These films then act as a carbon reservoir which upon heat treatment release carbon in the IF steel and strengthen it.
Coated tensile coupons 200 μm thick were annealed at different temperatures under high vacuum. Tensile tests show that a 100 MPa increase in yield stress can be obtained after annealing at 430 ˚C for 1h in high vacuum. The effects of annealing environment, film thickness and prestrain on carbon diffusion were also investigated. It was shown that carbon diffusion from the film to the IF steel substrate is limited by the film transformation into cementite at temperatures equal or higher than 530 ˚C.
All tensile curves showed a plastic instability known as Lüders plateau, which is undesirable as it results in surface markings on the deformed part. FEM analyses were performed to find ways to suppress the Lüders plateau, proving that increasing strain-hardening or having a graded instead of uniform carbon content through thickness can suppress or limit Lüdering.
The possibility of creating a through thickness gradient of microstructure was investigated as it could suppress Lüdering and result in higher strength. For these tests, FeC coated coupons were induction heated to 820 ˚C followed by water quenching. After only 2 minutes of heat treatment the yield stress was increased by 250 MPa and the ultimate tensile strength reached 400 MPa. With an annealing of 4 minutes, the Lüders plateau was fully suppressed and the microstructure consisted in ferrite grains and TiC nanocarbides. This work demonstrates that FeC films can be effectively used to diffuse carbon into steel and that a significant increase in mechanical properties can be obtained after a heat treatment of only a few minutes.
10
Alkhamis, A. T. A. "Structural optimization for static and free vibration conditions using genetic and gradient-based algorithms." Thesis, Swansea University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.635759.
Анотація:
This thesis is concerned with the development and application of reliable, creative and efficient computational tools for the analysis and structural optimization of 2D and 3D trusses under static and free vibration conditions and prismatic structures under static and free vibration conditions. The first part of the thesis deals with the analysis and optimization of 2D and 3D trusses under static and free vibration conditions. The second part of the thesis deals with prismatic structures under static and free vibration conditions which are idealised as effectively one-dimensional problems. The procedures used to define the structural shape and incorporating automatic mesh generation are described and the basic formulations of curved, Mindlin-Reissner finite elements and finite strips are presented for prismatic shells. The accuracy of the elements and strips is verified using several benchmark tests. The basic algorithm of structural shape optimization is then presented and several examples are carried out. In both parts the optimization analysis is carried out using two different algorithms: sequential quadratic programming (SQP) and genetic algorithms (GAs). The optimized structures are not checked for buckling. Six main computer FORTRAN programs are developed, documented and tested.
11
Dogan, Eda. "Effects of large-scale free stream turbulence on a zero-pressure-gradient turbulent boundary layer." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/412650/.
Анотація:
The focus of this study is to investigate the characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of large-scale free-stream turbulence. Particular attention is given to scale interactions occurring within the turbulent boundary layer. The free-stream turbulence was generated by an active grid. The investigation was conducted as an experimental work using hot-wire anemometry and Particle Image Velocimetry. Large-scale structures occurring in the free-stream are shown to penetrate the boundary layer and increase the streamwise velocity fluctuations throughout. The near-wall peak amplitude of the streamwise velocity fluctuations are observed to increase with increasing turbulence level in the free-stream. This trend has been found analogous to high Reynolds number flows for increasing ? . Free-stream turbulence imposes an outer energy peak in the boundary layer and the energy level of this peak is observed to increase with increasing turbulence level. It is also shown that the large-scales dominating the outer region of the boundary layer have a modulating effect on the small-scales in the near wall region; this effect becomes more significant with increasing turbulence level in the free-stream. These added up to the analogy between the high Reynolds number flows and present study cases. The latest efforts in the field of high Reynolds number flow investigations are aimed towards predicting the near-wall turbulence using only the large-scale information input. Therefore, this analogy has encouraging implications towards generalising large-scale influences on the near-wall small scales. The study also presents results regarding the structural organisation inside the boundary layer using the PIV data. The coherent structures found inside the boundary layer are observed to have inclined features as consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organisation of the structures inside the boundary layer could potentially provide an evidence for a universal structure for all wall-bounded flows as also proposed from previous studies in the literature.
12
Shariati, Hossein. "Micromechanical study of PFZ in aluminum alloys." Thesis, KTH, Hållfasthetslära (Inst.), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-190115.
Анотація:
There are a number of experiments showing that the ductility of aluminum alloys decreases during age-hardening heat treatment. Observing the grains of age-hardened aluminum alloys at the micron scale, one can notice that there are precipitate-free zones (PFZs) along the grain boundaries. PFZ has yield stress three times lower than the grain interior (bulk) due to absence of alloying elements. As a result, PFZ is suspected to be the reason for ductility reduction of alloys. On the other hand, a number of experiments performed on specimens with micron-scale dimensions have shown that the plastic deformation of crystalline materials is size-dependent. These micron-scale dimensions which can influence the mechanical behavior, such as yield stress or hardening, are not taken into account in the conventional plasticity theory, therefore another theory has been developed. That theory is Strain Gradient Plasticity (SGP). The specific SGP theory used here is a so called ‘higherorder theory’ in the sense that higher order stresses as well as additional boundary conditions are included in the theory. SGP theory also includes length scale parameters in order to be dimensionally consistent. On a recent study conducted by Fourmeau et al. (Fourmeau, 2015), transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of PFZ in the AA7075-T651 aluminum alloy. It is observed that the width of PFZ is about 20 to 40 nm. In the present thesis, the properties for PFZ and bulk material provided by that study are used for a micromechanical finite element model of a microstructure including the bulk, PFZ and the grain boundary (GB). A uniaxial loading condition is applied to the representative volume element (RVE) and SGP theory is hired in order to capture the plastic strain fields as well as the stress triaxiality in PFZ and bulk region. Moreover a damage criterion is employed and studied for models with PFZ and without PFZ to understand the role of PFZ in reduction of the ductility of aluminum alloys. It is found that the damage parameter is much higher in the presence of PFZ. Finally, the void growth is studied by adding voids at critical locations to the model.
13
Mao, Zirui. "A Novel Lagrangian Gradient Smoothing Method for Fluids and Flowing Solids." University of Cincinnati / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1553252214052311.
14
Dimitrijević, Bojan J. [Verfasser], Klaus [Gutachter] Hackl, and Paul [Gutachter] Steinmann. "On a regularization framework for inelastic material models via gradient enhancement of the free energy function / Bojan J. Dimitrijević ; Gutachter: Klaus Hackl, Paul Steinmann." Bochum : Ruhr-Universität Bochum, 2017. http://d-nb.info/1129452247/34.
15
Chirat, Mathieu. "Synthèse de nouveaux tensioactifs macromoléculaires complexants et étude de leurs interactions avec le cobalt pour le développement d’un procédé de décontamination des textiles en milieu CO2 dense." Thesis, Montpellier, Ecole nationale supérieure de chimie, 2012. http://www.theses.fr/2012ENCM0018/document.
Анотація:
Cette étude porte sur la décontamination de matrices textiles en milieu CO2 dense (CO2 liquide ou CO2 supercritique). Elle s'inscrit dans le cadre de la décontamination des textiles utilisés dans l'industrie nucléaire. Le CO2 dense est proposé comme alternative au milieu aqueux utilisé dans le procédé actuel et qui génère une importante quantité d'effluents aqueux contaminés nécessitant un post-traitement. Le contaminant étudié est le cobalt qui peut se présenter sous forme ionique ou particulaire. L'extraction du cobalt en milieu CO2 dense est assurée par un additif : un tensioactif macromoléculaire CO2-phile/CO2-phobe complexant. Plusieurs familles d'additifs ont été synthétisées par polymérisation radicalaire contrôlée : des copolymères à gradient comportant des motifs CO2-philes, siliciés ou fluorés, et des motifs CO2-phobes complexants de types acétoacétoxys, diéthylphosphonates ou acides phosphoniques. Le comportement de ces copolymères dans le CO2 dense a été évalué grâce à la détermination des diagrammes de phases copolymère-CO2 (par la mesure du point de trouble) et grâce à l'étude de leur autoorganisation dans le CO2 dense (par diffusion de neutrons aux petits angles). Les copolymères fluorés se sont avérés être les plus avantageux en termes de solubilité. Néanmoins, les copolymères siliciés présentent une solubilité compatible avec le procédé et ils constituent donc une alternative intéressante pour éviter la présence de fluor gênant pour le conditionnement des déchets nucléaires. L'étude de la complexation du cobalt par les copolymères (par spectrométrie UV-visible et par torche à plasma couplée à un spectromètre d'émission atomique) a permis d'établir des relations entre le type de motif complexant et l'affinité avec le cobalt. La solubilité dans le CO2 dense de ces complexes copolymères-cobalt est comparable à celle des copolymères seuls. De plus, l'étude de l'auto-organisation en milieu CO2 dense a révélé un faible taux d'agrégation des complexes copolymères-cobalt. Enfin, les copolymères synthétisés ont été mis en oeuvre dans les procédés de décontamination particulaire et ionique. Dans le cas du procédé de décontamination ionique, l'emploi du copolymère à gradient poly(acrylate de 1,1,2,2-tétrahydroperfluorodécyle-co-diacide vinylbenzylphosphonique) a permis d'atteindre environ 70% de décontamination grâce à la formation d'une microémulsion d'eau dans le CO2 dense. L'efficacité du procédé dedécontamination a été portée à 97% grâce à l'emploi de pyridine comme tiers additifThis study is about textile decontamination in dense CO2 (liquid CO2 or supercritical CO2). The study is carried out in the framework of decontamination of textile used in the nuclear industry. The dense CO2 offers an alternative to aqueous medium used in the current process which generates a huge quantity of contaminated aqueous effluent requiring a post-treatment. Cobalt is the targeted contamination and can be found as ionic species or particles. The cobalt extraction in dense CO2 is achieved with an additive : a complexing CO2-philic/CO2-phobic macromolecular surfactant. Several types of additives were synthesized by controlled free radical polymerization : gradient copolymers made with CO2-philic groups (silicone-based or fluorinated moieties) and CO2-phobic complexing groups (acetoacetoxy, diethylphosphonate or phosphonic acid moieties). The copolymer behavior in dense CO2 was determined by phase diagram measurements (cloud point method) and their self-assembly in dense CO2 was investigated by small angle neutron scattering. The fluorinated copolymers were found advantageous in terms of solubility. Nevertheless, the silicone-based copolymers showed solubilities which are compatible with the process, therefore they are a good alternative to avoid fluorinated compounds which are unwanted in the conditioning of nuclear wastes. The study of cobalt complexation by the copolymers (UV-vis spectroscopy and inductively coupled plasma-mass spectroscopy) established relations between the type of complexing group and the affinity with the cobalt. The solubility of copolymer-cobalt complexes in dense CO2 is similar to those of copolymers. Moreover, the self-assembly study of the complex revealed a low aggregation. Finally, the synthesized copolymers were used in particle or ionic decontamination processes. In the case of ionic decontamination process, a rate of 70% of decontamination was reached with the use of gradient copolymer poly(1,1,2,2-tetrahydroperfluorodecyle acrylate-covinylbenzylphosphonic diacid) which allowed the formation of water-in-CO2 microemulsion. The efficiency of the decontamination process was even improved up to 97% with the addition of pyridine in the process
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Valverde, Danillo Pires. "Estudo teórico de ácidos aminobenzóicos usando técnicas de mecânica quântica e simulação computacional." Universidade Federal de Goiás, 2016. http://repositorio.bc.ufg.br/tede/handle/tede/5557.
Анотація:
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Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq
In this work we realize a study of the aminobenzoic acid family (Abz) in neutral, anionic and cationic conformations combining a Sequential QM/MM method with mean field approximation (ASEC) with Free Energy Gradient method (FEG), for the solvents: cyclohexane, acetonitrile and water. In all cases, the Abz molecules type present a conformation in aqueous solution has greater geometry distortion compared to the gas phase. According to our results, there is just one cationic conformation in aqueous solution for ortho. For para and meta, anionic molecules the geometry is more stable planar conformation, which is a saddle point in gas phase. For neutral ortho, the rotamer I is the more energetically stable structure. In anionic conformations it is found the largest number of hydrogen bonds in aqueous solution. In acetonitrile and water, the energy stability of conformations can be explained by hydrogen bonds and solute molecule dipole moment. In all cases, the peak of the first absorption band is in good agreement with experimental results. Our results turn out to be more close to experimental result if explicit solvent molecules are included. The theoretical absorption spectra has similar behaviour to those obtained experimentally and the absorption band shifts are well described with ASEC, mainly those involving the neutral, anionic and cationic conformations in water.
Neste trabalho é realizado o estudo da família de ácidos aminobenzóicos (Abz) nas conformações neutra, aniônica e catiônica combinando o uso do método QM/MM sequencial com a aproximação de um campo eletrostático médio (ASEC) juntamente com o método de gradiente de energia Livre (FEG) para os solventes: ciclohexano, acetonitrila e água. Em todas as moléculas estudadas, a conformação em solução aquosa é a que apresenta maiores mudanças geométricas com respeito a fase gasosa. De acordo com os nossos resultados, existe apenas uma conformação catiônica em solução aquosa para o caso do orto. Nas moléculas aniônicas do p-Abz e do m-Abz a geometria mais estável é a conformação planar, que em fase gasosa corresponde a um ponto de sela. No caso do orto neutro, o rotâmero I é a estrutura mais favorável energeticamente. As conformações aniônicas em geral efetuam mais ligações de hidrogênios LH em meio aquoso. Em acetonitrila e água, a estabilidade energética é bem explicada pelas ligações de hidrogênio e pelo momento de dipolo da molécula do soluto. O pico da 1ª banda de absorção em todas as moléculas estudadas utilizando o ASEC se encontra em bom acordo com o experimental, mas com a inclusão de algumas moléculas explícitas do solvente os resultados obtidos se aproximam ainda mais do valor experimental. Os espectros de absorção teóricos possuem comportamento similar aos obtidos experimentalmente e os deslocamentos da banda de absorção são bem descrito com ASEC, principalmente os que envolvem as conformações neutra, aniônica e catiônica em água.
17
Silva, Carlos Eduardo Bistafa da. "Estudos teóricos do estado excitado de moléculas orgânicas em solvente." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-22102015-102635/.
Анотація:
Absorção e Emissão de radiação eletromagnética por moléculas na região do UV-Vis fornece informações sobre os estados eletrônicos excitados, sendo propriedades de grande interesse devido a sua relação com processos biológicos, bem como suas possíveis aplicações em diagnósticos e tecnologia. Essas propriedades são sensíveis ao meio em que as moléculas se encontram, tornando-se assim natural a busca por métodos teóricos que possibilitam descrever essas interações. Neste trabalho, nós usamos a metodologia Sequential-Quantum Mechanics/Molecular Mechanics para estudar o espectro de absorção e de emissão de moléculas de relevância biológica, quando em solução. Simulações clássicas Monte Carlo foram usadas pra construir uma configuração eletrostática média do líquido para posterior cálculo das propriedades, feito através dos métodos multiconfiguracionais CASSCF e CASPT2. Cuidados especiais foram tomados para incluir a polarização eletrônica que o soluto sofre devido à presença do solvente. Nossa contribuição é a adaptação do método do Gradiente de Energia Livre para permitir a obtenção de geometrias de estados excitados do soluto em solução. A técnica foi implementada em um programa e aplicada com sucesso nos sistemas estudados. As perspectivas agora se abrem para a obtenção de intersecções cônicas em meio solvente, permitindo assim o estudo de decaimentos não-radiativos em sistemas solvatados.Absorption and emission of electromagnetic radiation by molecules in the UV-Vis region yields information about the electronic excited states, being properties of great interest due its relation with biological process, as well as its possible applications in diagnostics and technology. These properties are sensible to the environment in which the molecules are, making natural the search for theoretical methods that describe these interactions. In this work, we have used the Sequential-Quantum Mechanics/Molecular Mechanics methodology to study the spectrum of absorption and emission of molecules of biological relevance, when in solution. Classical Monte Carlo simulations were carried out to construct an average solvent electrostatic configuration and used to represent the liquid in posterior quantum mechanics calculation of the properties, performed by using the multiconfigurational methods CASSCF and CASPT2. Special cares were taken to include the electronic polarization of the solute due the solvent. Our contribution is the adaptation of the Free Energy Gradient method to allow the determination of the solute excited states geometries in solution. The method was implemented in a computer program and successfully applied in the systems studied. The perspectives are now open to the calculation of conical intersection in solvent environment, enabling the study of nonradioactive decays in solvated systems.
18
Wycoff, Nathan Benjamin. "Gradient-Based Sensitivity Analysis with Kernels." Diss., Virginia Tech, 2021. http://hdl.handle.net/10919/104683.
Анотація:
Emulation of computer experiments via surrogate models can be difficult when the number of input parameters determining the simulation grows any greater than a few dozen. In this dissertation, we explore dimension reduction in the context of computer experiments. The active subspace method is a linear dimension reduction technique which uses the gradients of a function to determine important input directions. Unfortunately, we cannot expect to always have access to the gradients of our black-box functions. We thus begin by developing an estimator for the active subspace of a function using kernel methods to indirectly estimate the gradient. We then demonstrate how to deploy the learned input directions to improve the predictive performance of local regression models by ``undoing" the active subspace. Finally, we develop notions of sensitivities which are local to certain parts of the input space, which we then use to develop a Bayesian optimization algorithm which can exploit locally important directions.Doctor of Philosophy
Increasingly, scientists and engineers developing new understanding or products rely on computers to simulate complex phenomena. Sometimes, these computer programs are so detailed that the amount of time they take to run becomes a serious issue. Surrogate modeling is the problem of trying to predict a computer experiment's result without having to actually run it, on the basis of having observed the behavior of similar simulations. Typically, computer experiments have different settings which induce different behavior. When there are many different settings to tweak, typical surrogate modeling approaches can struggle. In this dissertation, we develop a technique for deciding which input settings, or even which combinations of input settings, we should focus our attention on when trying to predict the output of the computer experiment. We then deploy this technique both to prediction of computer experiment outputs as well as to trying to find which of the input settings yields a particular desired result.
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Herrera, Andrés Medina. "Estrutura eletrônica da amino- e dimetilamino-benzonitrila em meio usando métodos híbridos de QM/MM." Universidade Federal de Goiás, 2015. http://repositorio.bc.ufg.br/tede/handle/tede/5123.
Анотація:
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In this research we studied the structural and electronic properties of the ground state of molecules amino-benzonitrile (ABN) and dimethylamino-benzonitrile (DMABN), isolated and in different solvents.We performed computer simulations of those molecules in different solvents as cyclohexane, dichloromethane, acetonitrile and water. The structure electronic method MP2 (second order perturbation Møller-Plesset) was used to perform quantum calculations. To study the molecules in solvent we used the hybrid sequential QM/MM method combined with the free energy gradient method. The dual fluorescence to this type of molecules is a process that has been much studied but it is not well clarified that is the cause of the process. We performed the optimization of the molecules in an isolated state and in different solvents to determine the ground state structure. In the case of the DMABN molecule the optimization was performed both at room temperature and at low temperature, near the melting point of the solvent. We studied minimum energy point and some transition states of this molecules associated with the pyramidalization or the rotation of the amino group. The results showed that the molecules are pyramidal when they are isolated, and that in polar solvent they became less pyramidal. The rotation of amino group is unfavored in both molecules, increasing this effect in polar solvents.
Neste trabalho foram estudadas as propriedades estruturais e eletrônicas do estado fundamental das moléculas amino-benzonitrila (ABN) e dimetilamino-benzonitrila (DMABN), isoladas e em diferentes meios solventes. Foram realizadas simulações computacionais das moléculas em diferentes meios como, ciclohexano, diclorometano, acetonitrila e água. O método de estrutura eletrônica MP2 (Møller-Plesset em segunda ordem de perturbação) foi usado para fazer os cálculos quânticos. Para o estudo das moléculas em meio foi utilizado o método hibrido QM/MM sequencial combinado com o método de gradiente de energia livre. A dupla fluorescência para este tipo de moléculas é um processo que tem sido bastante estudado, mas ainda não está bem esclarecido qual é o causador do processo. Foram realizadas as otimizações das moléculas em estado isolado e nos diferentes meios, para determinar a estrutura do estado fundamental. No caso da molécula de DMABN a otimização foi feita tanto em temperatura ambiente como em baixas temperaturas, próximas do ponto de fusão dos solventes. Foram estudados pontos de mínimo e alguns estados de transição dessas moléculas associados à piramidalização ou à rotação do grupo amino. Os resultados mostram que essas moléculas são piramidais quando isoladas, e que em meio polar elas se tornam menos piramidais. A rotação do grupo amino é desfavorável em ambas as moléculas, aumentando esse efeito em meios polares.
20
Chen, Xiaodong. "Fluid-Structure Interaction Modeling of Epithelial Cell Deformation during Microbubble Flows in Compliant Airways." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1332208862.
21
Wang, Xiaoling. "MRI and NMR Investigations of Transport in Soft Materials and Explorations of Electron-Nuclear Interactions for Liquid-State Dynamic Nuclear Polarization." Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/75067.
Анотація:
The first part of this dissertation (Chapters 1 to 4) describes the use of magnetic resonance techniques for polymeric material characterizations in solutions, with emphasis on methods utilizing magnetic field gradients - magnetic resonance imaging (MRI) and pulsed-field-gradient (PFG) NMR. The second part (Chapter 5) presents enhancements to dynamic nuclear polarization, an intensity enhancement approach for magnetic resonance techniques.
In Chapter 2, I illustrate a characterization method to quantify free polymer chain content in a polymer/DNA complex (polyplex) formulation via one-dimensional proton NMR experiments. This assessment of free polymer quantity has critical impacts on in vivo gene transfection efficiency, cellular uptake, as well as toxicity of polycationic gene delivery vectors. Specifically, I investigated the complexation properties of three different polymeric "theranostic" agents, which combine an imaging functionality on the polymer as well as a DNA/RNA complexation component. These agents are under development to allow real time clinical monitoring of drug delivery and efficacy using MRI. Our NMR method provides simple and quantitative assessment of free and DNA-complexed polymers, including the actual polymer amine to DNA phosphate molar ratio (N/P ratio) within polyplexes. The NMR results are in close agreement with the stoichiometric number of polymer/DNA binding obtained by isothermal titration calorimetry. The noninvasive nature of this method allows broad application to a range of polyelectrolyte coacervates, for understanding and optimizing polyelectrolyte complex formation.
Chapter 3 demonstrates a time-resolved MRI approach for measuring diffusion of drug-delivery polymeric nanoparticles on mm to cm scales as well as monitoring nanoparticle concentration distribution in bulk biological hydrogels. Our results show that as the particle size and surface charge become larger, collagen gel at tumor relevant concentration (1.0 wt.%) presents a more significant impediment to the diffusive transport of negatively charged nanoparticles. These results agree well with those obtained by fluorescence spectroscopies (neutral or slightly positively charged diffusing particles) as well as the proposed electrostatic bandpass theory of tumor interstitium (negatively charged particles). This study provides fundamental information for the design of polymeric theranostic vectors and carries implications that would benefit the understanding of nanoparticle transport in solid tumors. Furthermore, this work takes a significant step toward developing quantitative and real time in vivo monitoring of clinical drug delivery using MRI.
Chapter 4 addresses the application of PFG-NMR for the determination of weight-average molar mass (Mw) for polyanions that have anti-HIV activity through the measurement of polymer diffusion coefficients in solutions. The effective characterization of molecular weights of polyelectrolytes has been a general and growing problem for the polymer industry, with no clear solutions in sight. In this study, we obtained the molar masses (Mw) for two series of sulfonated copolymers using sodium polystyrene sulfonate samples as molecular weight standards. PFG-NMR has notable advantages over conventional techniques for the characterization of charged polymers and shows great promise for becoming an effective alternative to chromatography methods.
Chapter 5 is devoted to experimental and theoretical studies of liquid state dynamic nuclear polarization (DNP) via the Overhauser effect. Based on the adventurous work done by previous Dorn group members, we show that for 1H-nuclide-containing systems, the dipolar DNP enhancement can be significantly improved by decreasing the correlation time of the interaction by utilizing a supercritical fluid (SF CO2) which allows for greater dipolar enhancements at higher magnetic fields. For molecules containing the ubiquitous 13C nuclide, we show that previously unreported sp hybridized (H-C) alkyne systems represented by the phenylacetylene-nitroxide system exhibit very large scalar-dominated enhancements. Furthermore, we show for a wide range of molecular systems that the Fermi contact interaction can be computationally predicted via electron-nuclear hyperfine coupling and correlated with experimental 13C DNP enhancements. For biomedical applications, the enhancement of metabolites in SF CO2 followed by rapid dissolution in water or biological fluids is an attractive approach for future hyperpolarized NMR and MRI applications. Moreover, with the aid of density functional theory calculations, solution state DNP provides a unique approach for studying intermolecular weak bonding interaction of solutes in normal liquids and SF fluids.Ph. D.
22
Zoltan, Pap. "Projektivni postupci tipa konjugovanih gradijenata za rešavanje nelinearnih monotonih sistema velikih dimenzija." Phd thesis, Univerzitet u Novom Sadu, Prirodno-matematički fakultet u Novom Sadu, 2019. https://www.cris.uns.ac.rs/record.jsf?recordId=110614&source=NDLTD&language=en.
Анотація:
U disertaciji su posmatrani projektivni postupci tipa konjugovanih gradijenata za rešavanje nelinearnih monotonih sistema velikih dimenzija. Ovi postupci kombinuju projektivnu metodu sa pravcima pretraživanja tipa konjugovanih gradijenata. Zbog osobine monotonosti sistema, projektivna metoda omogućava jednostavnu globalizaciju, a pravci pretraživanja tipa konjugovanih gradijenata zahtevaju maloračunarske memorije pa su pogodni za rešavanje sistema velikih dimenzija. Projektivni postupci tipa konjugovanih gradijenata ne koriste izvode niti funkciju cilja i zasnovani su samo na izračunavanju vrednosti funkcije sistema, pa su pogodni i za rešavanje neglatkih monotonih sistema. Pošto se globalna konvergencija dokazuje bez pretpostavki o regularnosti, ovi postupci se mogu koristiti i za rešavanje sistema sa singularnim rešenjima. U disertaciji su definisana tri nova tročlana pravca pretraživanjatipa Flečer-Rivs i dva nova hibridna pravca tipa Hu-Stori. Formulisani su projektivni postupci sa novim pravcima pretraživanja i dokazana je njihova globalna konvergencija. Numeričke performanse postupaka testirane su na relevantnim primerima i poređene sa poznatim postupcima iz literature. Numerički rezultati potvrđuju da su novi postupci robusni, efikasni i uporedivi sa postojećim postupcima.Projection based CG methods for solving large-scale nonlinear monotone systems are considered in this thesis. These methods combine hyperplane projection technique with conjugate gradient (CG) search directions. Hyperplane projection method is suitable for monotone systems, because it enables simply globalization, while CG directions are efficient for large-scale nonlinear systems, due to low memory. Projection based CG methods are funcion-value based, they don’t use merit function and derivatives, and because of that they are also suitable for solving nonsmooth monotone systems. The global convergence of these methods are ensured without additional regularity assumptions, so they can be used for solving singular systems.Three new three-term search directions of Fletcher-Reeves type and two new hybrid search directions of Hu-Storey type are defined. PCG algorithm with five new CG type directions is proposed and its global convergence is established. Numerical performances of methods are tested on relevant examples from literature. These results point out that new projection based CG methods have good computational performances. They are efficient, robust and competitive with other methods.
23
Lang, Humblot Karine. "Etude et réalisation d'un magnétomètre à résonance magnétique nucléaire à polarisation dynamique pour les applications en forage pétrolier." Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10023.
Анотація:
Les magnetometres a resonance magnetique nucleaire bases sur le phenomene de polarisation dynamique sont des instruments tres adaptes a la mesure du champ magnetique terrestre dans les puits de forage petrolier, ce qui permet une datation assez precise des couches geologiques concernees. Le magnetometre contient un solvant protone avec des radicaux libres en solution afin de realiser la polarisation dynamique qui amplifie le signal nucleaire des protons inobservable sans ce dispositif. Le but de ce travail est d'ameliorer les performances du magnetometre en presence de gradients de champs magnetiques importants et de permettre son fonctionnement jusqu'a des temperatures relativement elevees. Le choix de la solution radicalaire est crucial. Les criteres preponderants sont l'obtention d'un radical stable presentant une structure hyperfine interne importante et une largeur de raie rpe fine. Nous avons developpe un modele plus performant de cette largeur de raie qui nous a permis de definir precisement les caracteristiques requises pour le radical et le solvant. Nous avons montre que la tenue en gradient de la solution est conditionnee par des temps de relaxation nucleaires courts et la correlation entre ces deux effets a ete formulee a l'aide d'un modele theorique en accord avec les resultats experimentaux. L'ensemble de ces criteres a conduit au choix de la solution de triglyme contenant le radical nitroxyde tmio. Cette solution stable et relativement resistante aux gradients de champ magnetiques est la plus performante pour notre application. Un nouvel oscillateur base sur cette solution a ete concu et caracterise. Une etude fondamentale complementaire et originale sur la relaxation intermoleculaire entre les protons du solvant et les spins electroniques des radicaux a structure hyperfine interne a ete developpee. Le comportement des densites spectrales selon une loi proportionnelle a la racine de la frequence de resonance nucleaire a ete observe dans divers domaines de frequence, ce qui a permis une determination precise de la constante de diffusion relative du couple radical-solvant
24
Pouilly-Cathelain, Maxime. "Synthèse de correcteurs s’adaptant à des critères multiples de haut niveau par la commande prédictive et les réseaux de neurones." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASG019.
Анотація:
Cette thèse porte sur la commande des systèmes non linéaires soumis à des contraintes non différentiables ou non convexes. L'objectif est de pouvoir réaliser une commande permettant de considérer tout type de contraintes évaluables en temps réel.Pour répondre à cet objectif, la commande prédictive a été utilisée en ajoutant des fonctions barrières à la fonction de coût. Un algorithme d'optimisation sans gradient a permis de résoudre ce problème d'optimisation. De plus, une formulation permettant de garantir la stabilité et la robustesse vis-à-vis de perturbations a été proposée dans le cadre des systèmes linéaires. La démonstration de la stabilité repose sur les ensembles invariants et la théorie de Lyapunov.Dans le cas des systèmes non linéaires, les réseaux de neurones dynamiques ont été utilisés comme modèle de prédiction pour la commande prédictive. L'apprentissage de ces réseaux ainsi que les observateurs non linéaires nécessaires à leur utilisation ont été étudiés. Enfin, notre étude s'est portée sur l'amélioration de la prédiction par réseaux de neurones en présence de perturbations.La méthode de synthèse de correcteurs présentée dans ces travaux a été appliquée à l’évitement d’obstacles par un véhicule autonomeThis PhD thesis deals with the control of nonlinear systems subject to nondifferentiable or nonconvex constraints. The objective is to design a control law considering any type of constraints that can be online evaluated.To achieve this goal, model predictive control has been used in addition to barrier functions included in the cost function. A gradient-free optimization algorithm has been used to solve this optimization problem. Besides, a cost function formulation has been proposed to ensure stability and robustness against disturbances for linear systems. The proof of stability is based on invariant sets and the Lyapunov theory.In the case of nonlinear systems, dynamic neural networks have been used as a predictor for model predictive control. Machine learning algorithms and the nonlinear observers required for the use of neural networks have been studied. Finally, our study has focused on improving neural network prediction in the presence of disturbances.The synthesis method presented in this work has been applied to obstacle avoidance by an autonomous vehicle
25
Zhang, Yumeng. "Modélisation et simulation des dispositifs de ventilation dans les stockages de déchets radioactifs." Thesis, Nice, 2015. http://www.theses.fr/2015NICE4132/document.
Анотація:
L'objectif de cette thèse est de fournir des modèles et des outils de simulation pour décrire les échanges de masse entre les circuits de ventilation (galeries) et les milieux poreux des ouvrages souterrains d'enfouissement des déchets nucléaires. La modélisation prend en compte le couplage à l'interface poreux-galerie entre les écoulements liquide gaz compositionnels dans le milieu poreux constituant le stockage et les écoulements gazeux compositionnels dans le milieu galerie libreThe objective of this thesis is to develop models and algorithms to simulate efficiently the mass exchanges occurring at the interface between the nuclear waste deep geological repositories and the ventilation excavated galleries. To model such physical processes, one needs to account in the porous medium for the flow of the liquid and gas phases including the vaporization of the water component in the gas phase and the dissolution of the gaseous components in the liquid phase. In the free flow region, a single phase gas free flow is considered assuming that the liquid phase is instantaneously vaporized at the interface. This gas free flow has to be compositional to account for the change of the relative humidity in the free flow region which has a strong feedback on the liquid flow rate at the interface
26
Li, Qian. "Three-dimensional analysis of functionally graded material plates, free vibration in thermal environment and thermal buckling." Thesis, University of Macau, 2008. http://umaclib3.umac.mo/record=b1783646.
27
Li, Qi. "Acoustic noise emitted from overhead line conductors." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/acoustic-noise-emitted-from-overhead-line-conductors(90a5c23c-a7fc-4230-bbab-16b8737b2af2).html.
Анотація:
The developments of new types of conductors and increase of voltage level have driven the need to carry out research on evaluating overhead line acoustic noise. The surface potential gradient of a conductor is a critical design parameter for planning overhead lines, as it determines the level of corona loss (CL), radio interference (RI), and audible noise (AN). The majority of existing models for surface gradient calculation are based on analytical methods which restrict their application in simulating complex surface geometries. This thesis proposes a novel method which utilizes both analytical and numerical procedures to predict the surface gradient. Stranding shape, proximity of tower, protrusions and bundle arrangements are considered within this model. One of UK National Grid's transmission line configurations has been selected as an example to compare the results for different methods. The different stranding shapes are a key variable in determining dry surface fields. The dynamic behaviour of water droplets subject to AC electric fields is investigated by experiment and finite element modelling. The motion of a water droplet is considered on the surface of a metallic sphere. To understand the consequences of vibration, the FEA model is introduced to study the dynamics of a single droplet in terms of phase shift between vibration and exciting voltage. Moreover, the evolution of electric field within the whole cycle of vibration is investigated. The profile of the electric field and the characteristics of mechanical vibration are evaluated. Surprisingly the phase shift between these characteristics results in the maximum field occurring when the droplet is in a flattened profile rather than when it is ‘pointed’.Research work on audible noise emitted from overhead line conductors is reviewed, and a unique experimental set up employing a semi-anechoic chamber and corona cage is described. Acoustically, this facility isolates undesirable background noise and provides a free-field test space inside the anechoic chamber. Electrically, the corona cage simulates a 3 m section of 400 kV overhead line conductors by achieving the equivalent surface gradient. UV imaging, acoustic measurements and a partial discharge detection system are employed as instrumentation. The acoustic and electrical performance is demonstrated through a series of experiments. Results are discussed, and the mechanisms for acoustic noise are considered. A strategy for evaluating the noise emission level for overhead line conductors is developed. Comments are made on predicting acoustic noise from overhead lines. The technical achievements of this thesis are summarized in three aspects. First of all, an FEA model is developed to calculate the surface electric field for overhead line conductors and this has been demonstrated as an efficient tool for power utilities in computing surface electric field especially for dry condition. The second achievement is the droplet vibration study which describes the droplets' behaviour under rain conditions, such as the phase shift between the voltage and the vibration magnitude, the ejection phenomena and the electric field enhancement due to the shape change of droplets. The third contribution is the development of a standardized procedure in assessing noise emission level and the characteristics of noise emissions for various types of existing conductors in National Grid.
28
Liou, Cheng-Yeh, and 劉承曄. "HDR Deghosting Using Motion-Registration-Free Fusion in Luminance Gradient Domain." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/9pqrq2.
Анотація:
碩士國立臺灣大學
電子工程學研究所
106
Synthesizing a high dynamic range (HDR) image from multiple images with different exposure times has been widely adopted in the field of digital photography; however, it could be challenging when there are moving objects in the scenes. HDR deghosting methods are those which not only create HDR images but also remove ghosting effects caused by moving objects. Unlike static HDR imaging methods, HDR deghosting methods are not often used by professional or amateur photographers because the processing time is long and final HDR qualities are far from satisfactory. In this thesis, without applying time-consuming motion registration required by other HDR methods, the proposed algorithm has shorter running time. Additionally, a new moving region detection method is introduced. By performing color transfer right after exposure transfer, the input images for moving object detection stage show better exposure consistency as if they are captured under the same camera setting, thus producing better moving region detection result. In conclusion, this thesis demonstrates a novel and time-efficient algorithm which produces high quality HDR images for the scenes that have moving objects. Besides, the time-efficient feature makes it suitable for further HDR applications like HDR video or real-time preview of HDR images.
29
Kiraz, Fatma. "Distance Measurement-Based Cooperative Source Localization: A Convex Range-Free Approach." Thesis, 2013. http://hdl.handle.net/10012/7709.
Анотація:
One of the most essential objectives in WSNs is to determine the spatial coordinates
of a source or a sensor node having information. In this study, the problem of range
measurement-based localization of a signal source or a sensor is revisited. The main challenge of the problem results from the non-convexity associated with range measurements
calculated using the distances from the set of nodes with known positions to a xed sen-
sor node. Such measurements corresponding to certain distances are non-convex in two
and three dimensions. Attempts recently proposed in the literature to eliminate the non-
convexity approach the problem as a non-convex geometric minimization problem, using
techniques to handle the non-convexity.
This study proposes a new fuzzy range-free sensor localization method. The method
suggests using some notions of Euclidean geometry to convert the problem into a convex
geometric problem. The convex equivalent problem is built using convex fuzzy sets, thus
avoiding multiple stable local minima issues, then a gradient based localization algorithm
is chosen to solve the problem.
Next, the proposed algorithm is simulated considering various scenarios, including the
number of available source nodes, fuzzi cation level, and area coverage. The results are
compared with an algorithm having similar fuzzy logic settings. Also, the behaviour of
both algorithms with noisy measurements are discussed. Finally, future extensions of the
algorithm are suggested, along with some guidelines.
30
Chen, Chun-Hui, and 陳俊輝. "Design and Implementation of a Vehicle LED Front Fog Lamp with High Sharp Bright Gradient Based on a Free Form Surface Lens." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/26wnq6.
Анотація:
碩士國立虎尾科技大學
光電與材料科技研究所
102
Vehicle lighting systems, the lens''s primary purpose is to light converge or diverge, the vast majority of lenses are spherical lenses, but for some special purpose, such as non-elliptical reflector and the design of the shading plate under the lens to be alone to create a high gradient cutoff line, a spherical lens does not apply. Therefore, we need to design a specific lens to change the direction of light, and these lenses are aspheric mirror often or free-form surfaces, previously limited computing capacity and processing technical limitations, aspheric and free form lens design and production is very difficult, however, with the computing power and by ultra-precision processing technology development, aspheric and free form lens design and manufacturing is feasible and worth developing. This research in a free surface design method of the lens surface coated free LED light source, LED light collection angle of light of each in order to achieve high utilization of the results, was found by the experimental design, the use a single twin 425lm LED light source can meet the more stringent ECE R19 F3 regulations, and allows a uniform distribution of light shape, and in no case visor shield to create a high gradient cutoff line. In this study, the derivation of free-form surfaces, and using MATLAB calculates a free form lens, optical simulation software and then to run the simulation, and implementation verification, cancellation larger reflective surface energy and shading plate coated with LED lens designed simply to light design, not only can improve the utilization of light, reducing the number of LED used to better reduce the size of the optical element, and the reflecting surface eliminating the cost of processing and manufacturing processes.
31
Su, Yu-Ping, and 蘇育平. "Effect of temperature gradient on the growth of Ag3Sn intermetallic compounds in Pb-free solder during liquid-state and solid state aging process." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/38413774877087259321.
Анотація:
碩士國立清華大學
工程與系統科學系
102
Three-dimensional integrated circuit (3DIC) technology has become the major trend of electronic packaging in microelectronic industry. To effectively remove heat in stacked ICs, the temperature gradient must be established across the chips. Furthermore, because of the trend of even higher device current density, the joule heating would be more serious and the temperature gradient across the solder joints is expected to be larger. In this study, we investigated thermomigration behavior in SnAg solders during liquid-state and solid-state aging process, respectively. As compared to isothermal process, the Ag and Ag3Sn IMCs were dissolved near the hot end and asymmetric growth of Ag3Sn on both interfaces was observed. We proposed that asymmetric growth of Ag3Sn behaviors was due to diffusion of Ag toward the cold side under a temperature gradient, and thereby leading abnormal Ag3Sn IMCs growth at cold side. In addition, the corresponding growth mechanism of Ag3Sn IMC caused by thermomigration of Ag was discussed and growth rate of Ag3Sn IMCs at cold side was found to linearly increase with solid-aging time under a temperature gradient. The intrinsic parameter, heat of transport (Q*) of Ag, in molten-state and solid-state solders was calculated as +14.90 kJ/mole and +13.34 kJ/mole, respectively.
32
Amaral, Eliana Patrícia da Conceição. "Redox-free reverse electrodialysis using capacitive electrodes for energy generation." Master's thesis, 2020. http://hdl.handle.net/10362/113080.
Анотація:
The increasing demand for fossil fuels and arising concerns about their impact on the environment creates the need to develop sustainable technologies that make use of natural sources to generate energy, like reverse electrodialysis (RED) that makes use of salinity gradient power (SGP). In this thesis, a possible optimization of the electrode system applied in RED is introduced, including the preparation and study of capacitive slurry electrodes to replace redox electrodes commonly used.
Three slurries were prepared with different carbon concentrations (15 %, 20 %, and 22.4 %) in 1 M aqueous solution of NaCl. When pumped through a single membrane cell, it was possible to conclude about their resistance, and without being pumped their capacitance was obtained. The 22.4 %cwt slurry allowed the lowest electro-resistance at different conditions, while the 20 %cwt slurry achieved the highest specific capacitance of 34.52 F/g after 100 cycles. Pumping the slurries through the RED cell at 300 mL/min, while using the 20 %cwt slurry resulted in the best electrochemical performance, since the best trade-off between power density 1.56 W/m2, and energy efficiency 15.6 %, was achieved.
The results obtained demonstrate that it is possible to efficiently operate a redox-free RED cell. Further investigation on slurry preparation, charge percolation mechanism, and cell design, should be done aiming for complete validation of capacitive slurries application in RED, thus promoting its industrial application.
33
Devaraj, G. "Schemes for Smooth Discretization And Inverse Problems - Case Study on Recovery of Tsunami Source Parameters." Thesis, 2016. http://hdl.handle.net/2005/2719.
Анотація:
This thesis deals with smooth discretization schemes and inverse problems, the former used in efficient yet accurate numerical solutions to forward models required in turn to solve inverse problems. The aims of the thesis include, (i) development of a stabilization techniques for a class of forward problems plagued by unphysical oscillations in the response due to the presence of jumps/shocks/high gradients, (ii) development of a smooth hybrid discretization scheme that combines certain useful features of Finite Element (FE) and Mesh-Free (MF) methods and alleviates certain destabilizing factors encountered in the construction of shape functions using the polynomial reproduction method and, (iii) a first of its kind attempt at the joint inversion of both static and dynamic source parameters of the 2004 Sumatra-Andaman earthquake using tsunami sea level anomaly data. Following the introduction in Chapter 1 that motivates and puts in perspective the work done in later chapters, the main body of the thesis may be viewed as having two parts, viz., the first part constituting the development and use of smooth discretization schemes in the possible presence of destabilizing factors (Chapters 2 and 3) and the second part involving solution to the inverse problem of tsunami source recovery (Chapter 4).
In the context of stability requirements in numerical solutions of practical forward problems, Chapter 2 develops a new stabilization scheme. It is based on a stochastic representation of the discretized field variables, with a view to reduce or even eliminate unphysical oscillations in the MF numerical simulations of systems developing shocks or exhibiting localized bands of extreme plastic deformation in the response. The origin of the stabilization scheme may be traced to nonlinear stochastic filtering and, consistent with a class of such filters, gain-based additive correction terms are applied to the simulated solution of the system, herein achieved through the Element-Free Galerkin (EFG) method, in order to impose a set of constraints that help arresting the spurious oscillations. The method is numerically illustrated through its application to a gradient plasticity model whose response is often characterized by a developing shear band as the external load is gradually increased.
The potential of the method in stabilized yet accurate numerical simulations of such systems involving extreme gradient variations in the response is thus brought forth.
Chapter 3 develops the MF-based discretization motif by balancing this with the widespread adoption of the FE method. Thus it concentrates on developing a 'hybrid' scheme that aims at the amelioration of certain destabilizing algorithmic issues arising from the necessary condition of moment matrix invertibility en route to the generation of smooth shape functions. It sets forth the hybrid discretization scheme utilizing bivariate simplex splines as kernels in a polynomial reproducing approach adopted over a conventional FE-like domain discretization based on Delaunay triangulation. Careful construction of the simplex spline knotset ensures the success of the polynomial reproduction procedure at all points in the domain of interest, a significant advancement over its precursor, the DMS-FEM. The shape functions in the proposed method inherit the global continuity ( C p 1 ) and local supports of the simplex splines of degree p . In the proposed scheme, the triangles comprising the domain discretization also serve as background cells for numerical integration which here are near-aligned to the supports of the shape functions (and their intersections), thus considerably ameliorating an oft-cited source of inaccuracy in the numerical integration of MF-based weak forms. Numerical experiments establish that the proposed method can work with lower order quadrature rules for accurate evaluation of integrals in the Galerkin weak form, a feature desiderated in solving nonlinear inverse problems that demand cost-effective solvers for the forward models. Numerical demonstrations of optimal convergence rates for a few test cases are given and the hybrid method is also implemented to compute crack-tip fields in a gradient-enhanced elasticity model.
Chapter 4 attempts at the joint inversion of earthquake source parameters for the 2004 Sumatra-Andaman event from the tsunami sea level anomaly signals available from satellite altimetry.
Usual inversion for earthquake source parameters incorporates subjective elements, e.g. a priori constraints, posing and parameterization, trial-and-error waveform fitting etc. Noisy and possibly insufficient data leads to stability and non-uniqueness issues in common deterministic inversions. A rational accounting of both issues favours a stochastic framework which is employed here, leading naturally to a quantification of the commonly overlooked aspects of uncertainty in the solution. Confluence of some features endows the satellite altimetry for the 2004 Sumatra-Andaman tsunami event with unprecedented value for the inversion of source parameters for the entire rupture duration. A nonlinear joint inversion of the slips, rupture velocities and rise times with minimal a priori constraints is undertaken. Large and hitherto unreported variances in the parameters despite a persistently good waveform fit suggest large propagation of uncertainties and hence the pressing need for better physical models to account for the defect dynamics and massive sediment piles.
Chapter 5 concludes the work with pertinent comments on the results obtained and suggestions for future exploration of some of the schemes developed here.